This invention relates to dip coating, and more particularly to an apparatus and method for recovering excess coating material to reduce waste and reduce emission of volatile organic compounds.
Dip coating systems and apparatus are well known in the prior art. However, the need continues for systems and methods improving upon desired performance properties and lowering application costs. Increasingly both manufacturers and customers have been concerned with the environmental impact of products and production methods.
The environmental impact of dip coating systems stems from the composition of the coating materials and the volatile organic compounds (VOCs) that are emitted in the coating process. Improvements in coating materials resulting in lower VOC content have only partially addressed the problem. The need to protect products from corrosion, particularly metals such as iron and steel, may necessitate the continued use of dip coating materials containing VOCs and the continued need to further control of emissions of VOCs.
Additionally, dip coating systems may pose potential inefficiencies and workplace hazards. Personnel working near these dip coating systems take steps to ensure dripping coating material is captured or contained to the extent possible. Coating material dripping from a coated article may fall onto the factory floor causing waste, may require clean-up, and may damage the workplace. In facilities with multiple dip coating systems, the hazard is compounded with the risk of coating material dripping from a piece in one coating line or system contaminating the coating material and process in an adjacent coating line.
Dip coating systems of the prior art have not adequately addressed these concerns. A common solution previously used was a drip tray or drip sheet to capture excess coating material dripping from coated articles. One prior dip tank is shown in FIG. 1 where an article would be dipped in a tank below the surface of a coating material and subsequently positioned over a drip tray that captured dripping coating material. In the dip tank of FIG. 1, the drip tray was attached to one side of the tank; however, in the past other dip tanks have used a free standing or mobile drip tray or sheet (See U.S. Pat. No. 5,902,402). Although previous drip trays or sheets may have addressed some of the containment concerns, coating material drying on these drip trays or sheets created waste that had to be disposed of in accordance with strict government regulations. Moreover, the created waste itself as well as VOCs released during drying exacerbated the environmental impact of the process and reduced the capacity of a dip coating line operated under an emission permit setting VOC limits. The dried waste and the emissions had to be counted within the emission limits of VOCs under the permit.
Moving the coated article to a separate drip station (See U.S. Pat. No. 6,837,933) only exacerbated the problem since there was additional waste and VOC production in the transfer, as well as increased risk of contaminating adjacent systems. An alternative to moving the coated article was holding the coated article above the dip tank. This solution reduced the risk of contamination in transfer, but reduced the production capacity of the line and increased the coating costs, since significant drying time was required.
The present disclosure substantially reduces coating waste that must be considered in dip coating given products, and in turn markedly increases the production capacity of a dip coating line operating under given emission permits. A dip coating apparatus is disclosed comprising:
a container having opposing sides and a volume capable of holding a liquid coating material and at least one article to be coated by the liquid coating material, and a stepped apron adjacent at least one of the opposing sides of the container,
the stepped apron capable of holding the liquid coating material to a depth sufficient to maintain the coating material in the liquid state and receiving the liquid coating material dripping from at least one coated article positioned above the stepped apron; and
at least one support member above the container capable of holding at least one article to be coated by dipping in the container.
The liquid coating material may circulate over the stepped apron and the container may have stepped aprons adjacent both opposing sides. The support member may comprise a crane or alternatively a conveyer. The support member may be capable of transferring a coated article from a position over the container to a position over the stepped apron. Moreover, the support member may comprise a rack capable of supporting a coated article over the stepped apron.
This dip coating apparatus markedly increases the production capacity of a coating line. The amount of waste coating material and VOCs that must be counted in the line's emission permit is substantially reduced for a given amount of coated production. The amount of coating waste is limited to splashing and incidental losses. Most of the coating material that drips from the coated articles during drying is returned to and maintained in the coating container in liquid state in its stepped apron, and at the same time the amount of VOC emissions during the drying operation is reduced if not eliminated. There will be an increase in VOCs emitted from the surface of the containers with the stepped apron, because of the larger surface area of the liquid coating material, but the net amount of waste coating material and VOCs that must be counted in the emission limits of the environmental permit will be markedly reduced, and the production capacity of the dip coating apparatus will be correspondingly increased.
Also disclosed is a method of coating an article comprising the steps of:
assembling a container having opposing sides and a volume capable of holding a liquid coating material and at least one article to be coated by the liquid coating material, and a stepped apron adjacent at least one of the opposing sides of the container,
filling the stepped apron with the liquid coating material to a depth sufficient to maintain the coating material in the liquid state and receiving the liquid coating material dripping from at least one coated article positioned above the stepped apron; and
using the support member to lower an article into the container to coat the article with the coating material;
lifting the coated article above the liquid coating material surface using the support member; and
holding the coated article over the stepped apron for a decided period.
The method of coating an article may also comprise transferring the coated article from a position over the container to a position over the stepped apron. The step of transferring the coated article may be accomplished be a crane or alternatively by a conveyer.
The step of holding the coated article over the stepped apron may be accomplished by a crane or by a rack. The step of assembling the coating apparatus may also comprise stepped aprons adjacent both opposing sides of the container. The method of coating an article may also comprise holding the coated article over the second stepped apron for a decided time or circulating the liquid coating material over the stepped apron.
The method of dip coating an article may further comprise the steps of assembling a cooling system comprising a conveyor and at least one fan and cooling the article to a temperature less than about 200° F. (about 93° F.) before lowering an article into the container by using the support member to coat the article with the coating material. These steps are particularly useful where the method involves increasing the temperature of at least portions of the articles by, for example, welding or heat treating. These steps enable the application of a more uniform coating on the article since spot welds and other locally heated areas are cooled before coating.
The step of assembling a cooling system may comprise providing a plurality of rollers capable of supporting at least one article, and positioning each fan between two rollers to direct the airflow between the rollers. Also, prior to the cooling step, the method may further comprise the step of transferring the article by the conveyor to a position adjacent the fans.